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Re: Chabay/Sherwood



Well said...just one small point...the text you are referring to is no
longer in print...the new page number is 633 for anyone who is trying to
follow this conversation with the new text.

On Fri, 15 Feb 2002, John Mallinckrodt wrote:

On Fri, 15 Feb 2002, Ludwik Kowalski wrote:

I started reading "Electric & Magnetic Interactions" of Chabay
and Sherwood. I need some help.

1) ON PAGE 208 (Section 6.2.1) THEY SAY:

Now consider a circuit consisting of our mechanical "battery"
[Van de Graaff] and a nichrome wire that has some twists and
turns in it. You know from your own experiments that such
twists and turns do not seem to affect the amount of steady-
state current, which seems odd.

WHY SHOULD IT SEEM ODD?

Ludwik,

Are you working through the book or just reading it casually? It
seems to me that it you have actually done the exercise on the
previous page, you can hardly fail to understand why it seems odd.

To be specific, C&S show you a pair of well-separated,
vertically-oriented capacitor plates with charges distributed on
them as might normally be expected. There is a wire running from
the left side of the (positively charged) left plate around
underneath the capacitor to the right side of the (negatively
charged) right plate. The wire is shown with NO charges on it but
it has small "x"s at locations near its ends and its middle.

C&S ask the student to "draw vectors representing the approximate
electric field DUE TO ONLY THE CHARGES ON THE METAL PLATES." (My
caps replace their italics but both are intended to convey
emphasis.) They go on to say, "In a different color, indicate the
drift velocity of the electron sea at those locations, due to
those electric fields." (This *is* page 207 in the book and the
student has learned a few things by now.) The point is that the
field from the charges on the capacitor would be expected--all by
itself--to drive the electrons in the appropriate direction.

After some more discussion, C&S show the wire making a (reverse)
S curve below the capacitor plates and ask the student to do the
same exercise. Now, however, we see electrons moving in ways that
don't make sense--i.e., that seem odd. In particular, electrons
in all three legs are moving to the left!!

2) REFERRING TO THE FIGURE ON PAGE 208 (PAGE 240 SHOWS IT
AGAIN WITH MORE DETAILS) THEY WRITE:

Good greef! We've got the electron current running upstream
through part of the wire!

I SUPPOSE THAT FOR ELECTRONS THE WORD "UPSTEAM" MEANS "IN THE
DIRECTION OF E." WHICH "PART OF THE WIRE" IS IT?

No. C&S are referring to the fact that the diagram on page 240
shows electrons moving opposite the expected direction of flow
along the wire from the negative plate to the positive plate. Look
at the "x" in the middle of the S-curve. They point out that the
diagram on page 240 violates the assumed steady-state condition
(since electrons will pile up on the "left bend" and be depleted
on the right bend.)

3) REFERRING TO THE "LEFT BEND" ON THE SAME FIGURE THEY WRITE:

Take a look at the left bend of the wire. If the only charges
were on the plates ["battery" terminals ?] electron current
would flow toward the bend from both neighboring sections.

I HAVE NO IDEA WHY WOULD ELECTRONS CONVERGE ON THE LEFT BEND
FROM BOTH SIDES DUE TO THE CHARGES ON THE PLATES.

Then I think you simply need to work though the previous exercises
a little more slowly. I don't understand why you have no idea
about that.

I WOULD EXPECT ELECTRONS TO ENTER THE LEFT BEND FROM ABOVE AND
LEAVE IT FROM BELOW. CHARGES ON THE PLATES CREATE A FIELD
"PUSHING ELECTRONS". WHAT AM I MISSING IN THEIR EXPLANATIONS?

? Do you have this reversed? You should "expect" electrons to
enter from below and leave from above as they move from the
negative plate to the positive plate.

John Mallinckrodt mailto:ajm@csupomona.edu
Cal Poly Pomona http://www.csupomona.edu/~ajm


Joseph J. Bellina, Jr. 219-284-4662
Associate Professor of Physics
Saint Mary's College
Notre Dame, IN 46556